The present invention relates to a process for producing polymer fibers, especially nano- and mesofibers, by the electrospinning process, and to fibers obtainable by this process.
For the production of nano- and mesofibers, a multitude of processes are known to those skilled in the art, among which electrospinning is currently of the greatest significance. In this process, which is described, for example, by D. H. Reneker, H. D. Chun in Nanotech. 7 (1996), page 216 ff., a polymer melt or a polymer solution is exposed to a high electrical field at an edge which serves as an electrode. This can be achieved, for example, by extrusion of the polymer melt or polymer solution in an electrical field under low pressure by a cannula connected to one pole of a voltage source. Owing to the resulting electrostatic charge of the polymer melt or polymer solution, there is a material flow directed toward the counterelectrode, which solidifies on the way to the counterelectrode. Depending on the electrode geometries, nonwovens or assemblies of ordered fibers are obtained by this process.
DE-A1-101 33 393 discloses a process for producing hollow fibers with an internal diameter of from 1 to 100 nm, in which a solution of a water-insoluble polymer—for example a poly-L-lactide solution in dichloromethane or a polyamide-46 solution in pyridine—is electrospun. A similar process is also known from WO-A1-01/09414 and DE-A1-103 55 665
DE-A1-196 00 162 discloses a process for producing lawnmower wire or textile fabrics, in which polyamide, polyester or polypropylene as a thread-forming polymer, a maleic anhydride-modified polyethylene/polypropylene rubber and one or more aging stabilizers are combined, melted and mixed with one another, before this melt is melt-spun.
The electrospinning of polymer melts allows only fibers of diameters greater than 1 μm to be produced. For a multitude of applications, for example filtration applications, however, nano- and/or mesofibers having a diameter of less than 1 μm are required, which can be produced with the known electrospinning processes only by use of polymer solutions.
However, these processes have the disadvantage that the polymers to be spun first have to be brought into solution. For water-insoluble polymers, such as polyamides, polyolefins, polyesters or polyurethanes and the like, nonaqueous solvents—regularly organic solvents—therefore have to be used, which are generally toxic, combustible, irritant, explosive and/or corrosive.
In the case of water-soluble polymers, such as polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, hydroxypropylcellulose and the like, it is possible to dispense with the use of nonaqueous solvents. However, fibers obtained in this way are by their nature water-soluble, which is why their industrial use is very limited. For this reason, these fibers have to be stabilized toward water after the electrospinning by at least one further processing step, for example by chemical crosslinking, which constitutes considerable technical complexity and increases the production costs of the fibers.